The invention relates to methods and devices for producing a tubular bag, for instance a so-called stickpack bag with laterally offset longitudinal seam.
The term “laterally offset longitudinal seam” here means that the seam region, in the case of a tubular bag delimited by transverse seams, lies outside the middle, i.e. the longitudinal median plane of the bag does not intersect the region of overlap of the web margins which form the longitudinal seam, for instance a so-called fin seam or a lap seam. In FIG. 1, the cross sections of traditional, symmetrical bag cross sections, in which the seam lies in or close to the middle of one side of the bag, are compared to the asymmetrical bag cross sections discussed here in which the longitudinal seam is arranged laterally, respectively in the embodiment comprising a lap seam and a fin seam.
The production of a tubular bag with laterally offset longitudinal seam can be realized by shaping the tube cross section from a flat packaging material web, for instance a film or paper web, by means of a forming shoulder or other suitable means, and sealing the longitudinal seam by a longitudinal welding member for welding together the web edges, and then subsequently closing the individual bags obtained from the thus generated tube by means of a transverse welding device.
At present, stickpack tubular bags, i.e. tubular bag portion packs of 8 to 50 mm bag width, are produced with a longitudinal seam located in or close to the middle of one side of the flat-pressed bag. The seam width in stickpacks is relatively large in relation to the bag width. The seam side of the bag has therefore only limited use for advertising and information purposes. In standard format tubular bags (50 to 250 mm), it is very common for these reasons to place the longitudinal seam on the side.
That this feature has so far yet to become established in the stickpack field is due to the fact that the subsequent treatment of the asymmetrical tubes in traditional bag forming, filling and sealing machines can only be realized with great effort, i.e. a special machine for these formats would have to be developed.
Unlike in bag forming, filling and sealing machines for standard formats, stickpack machines operate always on a multi-row basis, i.e. in the stickpack machine a plurality of stickpacks are shaped and filled next to one another. In tubular bags whose longitudinal seam is arranged in the middle of one side of the tube cross section, the longitudinal welding members, and, where necessary, counterstays, may be arranged such that they act transversely to the row-like arrangement, as is represented in FIG. 2.
If the longitudinal seam is to be arranged laterally offset, however, then too little space for the lateral positioning of the longitudinal welding device remains between the forming shoulders. Contrary to the solution in the standard machine for welding together of a symmetrical tube, the advancement of the welding tool would need to be realized at an oblique angle, as is represented in FIG. 3. This would be very complex, however, which is why there has hitherto been no mechanical realization of this arrangement.
The described technical difficulties which have so far prevented stickpack bags with laterally situated longitudinal seam from being made available on the market have to be surmounted.
The procedural problem in developing a forming shoulder for stickpack bags with laterally situated longitudinal seam consists in the fact that the web (of film or paper), due to the summarily greater forming and friction forces on the long side, slides away laterally. The developed line length of the web which is diverted on the forming shoulder is, starting from the median line, of varying length. The working resistance in the forming of the web on the forming shoulder is substantially determined by friction and deformation forces. This is intended to be compensated by designing the forming shoulder geometry such that these forces are equalized and the web is prevented from sliding away on one side.
The web which has once been shaped by the forming shoulder in the longitudinal direction tends to stretch out again after having been released. With respect to the longitudinal welding, there are technological risks involved in the positioning and fixing of the tube shortly before being welded. This is intended to be secured by optimal design of the forming shoulder and longitudinal welding device, i.e. longitudinal welding member and, where necessary, counterstay, and, where necessary, of further fixing members. For the solution to the problem of the asymmetrical, i.e. laterally offset longitudinal welding, methods and devices offering a completely new approach to the design of the forming shoulder and of the longitudinal welding device are proposed.
The device can here comprise a longitudinal welding member and a counterstay for the longitudinal welding member, wherein the counterstay extends past a tube-forming element of the forming shoulder, for instance the prism part of the forming shoulder and/or the filling pipe projecting into a forming shoulder of this type, or partially embraces said tube-forming element, and thus supports the longitudinal seam against the action of the longitudinal welding member. Such an embodiment of the proposed device is represented in FIG. 5. Alternatively, longitudinal welding members and counterstays can be arranged on the same side of the tube-forming element. They can in this case be movably connected to one another. Moreover, in both embodiments, the counterstay can also be configured as a longitudinal welding member.
Furthermore, the device can comprise a transverse welding device for compressing the tube and forming a transverse weld seam.
The method for producing the tubular bag with laterally offset longitudinal seam can comprise the following steps:
1. Shaping of a tube having a longitudinal seam, oriented perpendicularly to the median plane of the bag, from a planar material, for instance a film web,
2. Fixing, i.e. sealing of the longitudinal seam by longitudinal welding with a welding member which is configured as described above and acts against a counterstay,
3. Compression of the tube and, where necessary, formation of a transverse weld seam by means of a transverse welding device.
The effect of the third method step of the above-described embodiment is represented in FIG. 4.
The proposed device for producing tubular bags with laterally situated longitudinal seam can be fitted onto traditional bag forming, filling and sealing machines retrofitted without additional effort, i.e. pre-existing machines can be easily upgraded.
By means of a forming shoulder, an asymmetrical film tube cross section is produced and is generated by compression, while maintaining the working direction of the longitudinal welding tools in standard machines.
The traditional problems with respect to the skewing of the longitudinal welding members and, where present, the counterstays, are circumvented here. The solution can be implemented on standard machines. All that is needed is to laterally offset the longitudinal welding members and, where necessary, install an appropriate counterstay.
In order to surmount the above-described problems in the production of stickpacks with laterally offset longitudinal seam, in particular where a plurality of forming, welding and filling devices are provided, a device for producing a tubular bag, and a forming shoulder, which can be a component part of the described device, are proposed below.